The invention relates to a neutron detection device designed for measuring the neutron fluence rate outside the core of a nuclear reactor, this device comprising on the one hand at least one detector formed by an uranium-lined fission type ionization chamber having a positive electrode polarized by a positive voltage source and delivering an output signal applied to an electronic measuring circuit and a negative electrode connected to ground, this detector being housed in a containment containing a gas or a mixture of gases, and on the other hand at least one moderator element to slow down the fast neutrons.
A device of this type is described notably by the French Patent application published under the No. 2,670,301 filed by the applicant. In this device the moderator element is formed by an element made of highly hydrogenated material such as for example high-density polyethylene, placed inside the containment.
This material happens to be degraded by the action of radiation, which is liable to modify the functional characteristics considerably.